Question

I am creating widget classes that draw grids using QWidgets paint event. My base class BaseRenderArea contains this method:

//draws a list of guesses in the grid
void BaseRenderArea::drawGuesses(const QList <GuessPoint> &list, QPainter *painter)
{
    //QPainter painter1(this);

    QPen currentPen = painter->pen();

    painter->setPen(QString("red"));

    for(int i=0;i<list.size();i++)
    {
        GuessPoint gp = list.at(i);

        switch(gp.m_type)
        {
        case GuessPoint::Miss:
            drawMiss(gp.m_row, gp.m_col, painter);
            break;
        case GuessPoint::Dead:
            drawDead(gp.m_row, gp.m_col, painter);
            break;
        case GuessPoint::Hit:
            drawHit(gp.m_row, gp.m_col, painter);
            break;
        default:
            ;
        }
    }

    painter->setPen(currentPen);

}

I am calling this method from the paintEvent of the derived class widget.

//the paint event
//uses the base class to draw the texts and
//the grid
//and then draws the guesses made until now
//as well as the score for each choice of the computer
void DebugRenderArea::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);

    BaseRenderArea::paintEvent(event);

    //to do: draw guesses
    drawGuesses(&painter);

    //draw scores
    //drawScores(&painter);

}

The drawGuesses method in the derived class is implemented as follows:

//draws the guesses made until this moment
void DebugRenderArea::drawGuesses(QPainter *painter)
{
    QList <GuessPoint> list = m_logic->getListGuesses();
    BaseRenderArea::drawGuesses(list,painter);

}

This approach gives me a segmentation fault in the BaseRenderArea::drawGuesses in the second line painter->setPen….

If I change the BaseRenderArea::drawGuesses with the following code (not reusing the QPainter created in the derived class but creating a new QPainter):

//draws a list of guesses in the grid
void BaseRenderArea::drawGuesses(const QList <GuessPoint> &list, QPainter *painter)
{
    QPainter painter1(this);

    QPen currentPen = painter1.pen();

    painter1.setPen(QString("red"));

    for(int i=0;i<list.size();i++)
    {
        GuessPoint gp = list.at(i);

        switch(gp.m_type)
        {
        case GuessPoint::Miss:
            drawMiss(gp.m_row, gp.m_col, &painter1);
            break;
        case GuessPoint::Dead:
            drawDead(gp.m_row, gp.m_col, &painter1);
            break;
        case GuessPoint::Hit:
            drawHit(gp.m_row, gp.m_col, &painter1);
            break;
        default:
            ;
        }
    }

    painter1.setPen(currentPen);

}

everything works fine. My questions why ? What can I do to use the QPainter created in the paintEvent of the derived class in method of the base class?

Answer 1

The documentation says that you can’t have 2 active QPainters on the same paint device.

So, because BaseRenderArea::paintEvent is probably creating its own QPainter, it might have invalidated the one you created before in DebugRenderArea::paintEvent.

You could simply reorder the two lines:

BaseRenderArea::paintEvent(event);

QPainter painter(this);